Thermodynamic analysis of ore-forming conditions and sulfur isotopic systematics of Dachang ore field

Abstract

On the basis of ore-forming periods and stages of the Dachang ore field, the pH and$f_{0_2 }$ conditions and the S isotopic systematics during ore formation have been thcrmodynamically treated in this paper. Calculations show a progressively decreased pH, an increased oxidation regime and an intensified activity of sulfur from the early to the late stage. Owing to the unreliability of inferring the S source from δ³⁴S_min,$\delta ^{34} S_{\Sigma _S }$ has been calculated using the Ohmoto’s model. Results indicate that the δ³⁴ _min frequency distribution is more concentrated than that of δ³⁴S_min and the peak value shifts to negative region by 2.5%. The sulfur in the whole ore field seems to be of multiple source, i.e., different deposits have their own S sources. But the S isotopic composition pertaining to each stage is nearly constant, suggesting that the ore-forming system be open to sulfur and the supply of sulfur be sufficient. The conclusions deduced from calculations are supported by many lines of geological evidence.